In general, apparatus for controlling distance between vehicles have progressed to the state of an adaptive cruise control (ACC) system, which can control the speed of a vehicle at high running speeds in the presence of a preceding vehicle a long distance away. However, the distance to a closely preceding vehicle has a dynamic characteristic at low speeds, that is, a following vehicle must frequently accelerate and/or decelerate. Considering such a fact, realizing stable control of a vehicle speed is more difficult when a vehicle is running at low speed than at high speed.
Furthermore, the sensor for detecting distance to a preceding vehicle, which provides a key factor to determine whether a vehicle should be accelerated or decelerated, usually does not cover distances from a short range to a long range. A typical long range radar sensor usually has an applicable range of 10–150 m, and a short range radar sensor having a range of 0.2m˜20 m is only under investigation. Currently, a stable short range radar sensor is hard to find. In aspects of price and/or weight, such a short range radar sensor is not appropriate to be mounted together with a long range radar sensor.
For reference, in order to enable obtaining of a relative distance, a short range radar sensor is required to provide features of measuring distance and angle. A feature of measuring relative speed is further preferably required for controlling distance to a close object. A required range of the measuring is, e.g., 0.2˜20 m with respect to distance, and ±20 deg with respect to angle.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.